Core B will be responsible for the analysis of atmospheric 14C by Accelerator Mass Spectrometry (AMS) in samples of DNA collected from female and male human hearts as a function of age. Cardiomyocytes, endothelial cells (ECs), and fibroblasts will be evaluated separately to define their average age and turnover rate. Additionally, the levels of 14C-labeled or 3H-labeled thymidine in the various cardiac cell populations of mice and dogs injected with this isotope during the course of life will be part of the assays to be performed in this core. The delivery of the isotope will allow the measurement of 14C or 3H not only in cardiomyocytes, ECs, and fibroblasts, but also in smaller cell classes including vascular smooth muscle cells (SMCs) and cardiac stem cells (CSCs). By this approach, AMS will provide an independent value of the age and renewal of each cardiac cell category with aging and gender in animals and humans.